It can be foreseen that new battery systems, for example with rechargeable lithium-ion batteries, rechargeable lithium-polymer batteries or rechargeable nickel-metal hydride batteries, will increasingly be used as rechargeable electrical energy stores (EES) both in stationary applications, for example in wind power plants, and also in mobile applications, for example in electric motor vehicles (electric vehicles, EV) or hybrid vehicles (hybrid electric vehicles, HEV).
Rechargeable lithium-ion batteries comprise a positive electrode (cathode) and a negative electrode (anode), which electrodes can reversibly incorporate lithium ions (Li+) during charging (intercalation) or expel said lithium ions again during discharging (deintercalation). Rechargeable lithium-ion batteries have a high energy density and a low level of self-discharge.
The battery systems have to meet very stringent requirements in respect of the usable energy content, the degree of charging/discharging efficiency, the reliability, the service life and the undesired loss of capacity due to frequent partial discharging.
A battery system comprises a large number of battery cells. The battery cells heat up during charging and discharging on account of their cell internal resistance and the electrochemical processes which take place. The battery cells can be connected in series in order to increase the electrical voltage, and/or can be connected in parallel in order to increase the maximum electric current. In this case, the battery cells can be combined to form battery units or battery modules. By way of example, three to twelve battery cells can be combined in one battery module. The battery module holds the battery cells and absorbs mechanical stresses, and therefore it protects the battery cells against damage. Furthermore, the battery module can be used to control the temperature of the battery cells. In addition, the battery module can mechanically brace the battery cells and provide electrical insulation. The battery modules can be combined to form a battery pack. When used for driving vehicles, it is possible for, for example, approximately 100 battery cells (in the form of a traction battery) to be connected in series or in parallel. The total voltage can therefore be, for example, 450 V or even 600 V in a high-voltage battery system.
The temperature range which is permissible for operating the battery cells is typically between −30° C. and +70° C., preferably between +5° C. and +35° C. The performance of the battery cells can fall considerably in the lower range of the operating temperature. At temperatures below approximately 0° C., the internal resistance of the battery cells increases greatly, and the performance and the degree of efficiency of the battery cells continuously fall as temperatures fall further. Irreversible damage to the battery cells can also occur in the process. The performance of the battery cells can also fall considerably when the operating temperature is exceeded. The service life of the battery cells is reduced at temperatures over approximately 40° C. Irreversible damage to the battery cells can likewise occur in the process. Furthermore, the difference in temperature (temperature gradient), which is permissible for operating the battery cells, in a battery cell and/or within a battery module or a battery is typically between 5 Kelvins and 10 Kelvins. Different regions of a battery cell or different battery cells of a battery module or of a battery can be subject to different loadings or even be (partially) overloaded and/or damaged in the event of relatively large differences in temperature. Furthermore, there is a risk of condensation water forming in the battery on account of differences in temperature and/or changes in temperature. The damage can lead to accelerated aging of the battery cells or to thermal runaway of the battery cells, this posing a risk to people and the environment.
In a hybrid drive train of a vehicle, lithium-ion high-rupture-capacity battery cells are operated with very high dynamics. During brief peak loadings, which arise, for example, due to recuperation of braking energy during braking or boost support during acceleration, the battery cells have to absorb a high power within a very short time (during charging) or output a high power in a very short time (during discharging). On account of the internal resistance of the battery cells, these short peak loadings lead to significant heating of the battery cells. The degree of efficiency of the battery cells during charging and discharging is very high (approximately 95%); nevertheless, the waste heat which arises in the process is not negligible. At a traction power of, for example, 60 KW, a loss of 5% results in a loss of power of 3 KW. Furthermore, for example in the summer months or in warmer regions, external temperatures which may be 40° C. and more may lie outside the permissible temperature range, and therefore the battery cells cannot achieve the required service life of, for example, ten or 15 years without cooling.
In order to ensure the reliability, functioning and service life of the battery module or battery system, it is therefore necessary to operate the battery cells within the prespecified temperature range. Firstly, as described above, heat which has to be dissipated in order to prevent the battery cells heating up beyond the critical maximum temperature is produced during operation of the battery cells. Secondly, it may be necessary to heat up the battery cells to a minimum temperature at low temperatures. In order to maintain the prespecified temperature range, the temperature of the battery module or battery system is controlled, that is to say said battery module or battery system is cooled or heated according to requirements, with cooling generally being required more frequently than heating.
For this purpose, the battery module or battery system can comprise a fluid, for example a liquid, such as alcohol, for example propane-1,2,3-triol (glycerol, glycerine), oil or water, a liquid mixture or a coolant as the temperature-control medium, for example coolant or refrigerant in a temperature-control-medium circuit.
By way of example, the temperature of the battery cells can be controlled by temperature-control elements, for example temperature-control plates, on which the battery cells are fitted. By way of example, the temperature-control elements comprise an at least partially hollow body which is composed of a material with a high level of thermal conductivity, for example a metal such as aluminum, and connection ports which are composed of the same material and are soldered onto the body. In the temperature-control elements, for example either a coolant, such as cooling water (air/heat radiator) or a refrigerant, which evaporates due to the heat, (evaporator) absorbs the heat from the battery cells and dissipates said heat to the surrounding area or an air-conditioning system (air conditioning, AC) by means of a radiator or another heat-exchange device. In addition to the temperature-control elements or the evaporator and the heat exchanger or radiator, a temperature-control system further comprises tubes and/or pipes, for example which are composed of plastic or metal such as aluminum, for connecting the temperature-control elements, the evaporator, the heat exchanger and/or the radiator. Therefore, the connection ports which are composed of aluminum can for example be connected to a tube system which is composed of plastic. These connections can be subject to a great deal of stress under changing thermal and hydraulic conditions. However, very stringent requirements are placed on the leaktightness of the cooling system in this case.
DE 10 2011 082 991 A1 already discloses a battery comprising a battery housing, temperature-control elements and a lithium-ion cell which is in direct contact with one of the temperature-control elements.
In order to improve the functionality of battery modules and to reduce the costs of the battery modules, it is therefore necessary to allow further improvement of the temperature-control system.